Silver 3d pieces were made until 1945, although 1944 is the last year when many were issued. According to Wikipedia, many of the 1945 issue were melted down for their silver content, and were not widely circulated.
en.wikipedia.org
"Brass" 12-sided 3d pieces were made from 1937 until 1967, plus some 1970 coins in "proof sets" (for coin / souvenir collectors).
Some trial coins were made dated 1937, with the head of King Edward VIII, but he abdicated before they were issued. A handful "escaped" and are worth a small fortune if you find one.